1. Field of the Invention
This invention relates generally to a vehicle powertrain, and, in particular, to control of a transmission having no torque converter during a power-off to power-on transition.
2. Description of the Prior Art
A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts. A dual clutch transmission (DCT) has no torque converter
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
During a vehicle launch condition in a conventional vehicle whose powertrain includes a powershift transmission, the engine and transmission are concurrently controlled in a coordinated manner to provide acceptable vehicle launch performance. In a powershift transmission vehicle application, providing consistent and acceptable vehicle launch performance can be a rather difficult control problem due to the lack of a torque converter. During a vehicle launch condition in this type of vehicle application, the torque capacity of the transmission clutch and slip across the clutch are carefully controlled in coordination with the engine torque to provide the desired vehicle response.
During a power-off to power-on transition, if torque capacity is present at the active input clutch, a driveline disturbance due to clutch stick is likely to occur. Yet torque transmitting capacity through that clutch is required to prevent driveline disturbance associated with synchronizer engagements. Therefore, a need exists in the industry for a control that avoids driveline disturbance due to clutch stick during power-off to power-on transitions while the torque capacity of the active or oncoming input clutch is not zero.